A gas turbine engine consists of several components. During operation, the components of the gas turbine engine are typically exposed to harsh environments that can damage the turbine components. Environmental damage can occur in various modes, including damage as a result of heat, oxidation, corrosion, hot corrosion, erosion, wear, fatigue or a combination of several degradation modes.
Today's turbine engine is designed and operated in such a way that the environmental conditions and consequently the types of environmental damages in different regions of the various components of the turbine can vary significantly from one another. As a result, an individual turbine engine component often requires several coating systems to protect the underlying base materials of the component.
As an example, FIG. 1 shows the various sections of a typical turbine blade. The turbine blade has several sections, including a platform, an airfoil extending upwardly from the platform, a shank extending downwardly from the platform, a root extending downwardly form the shank, and internal cooling passages located insides the root, shank and airfoil. The platform has a top side adjacent to the airfoil and a bottom side adjacent to the shank.
In service, the airfoil and platform operate at the hottest regions of the turbine blades, and are therefore subject to oxidation degradation. Consequently, protection of the base materials of the airfoil regions and the top platform surface generally requires an oxidation-resistant coating, such as a diffusion aluminide coating and/or a MCrAlY overlay coating. These oxidation-resistant coatings are capable of forming a slowing-growing and adherent alumina scale. The scale provides a barrier between the metallic substrate and the environment. A thermal barrier coating can optionally be applied as top coat over the oxidation-resistant coating to further reduce metal temperature and increase service life of the component.
In contrast to the airfoil and platform, the other regions of the turbine blade, including the regions under the platform, shank, root and internal cooling passages, are exposed during service to relatively lower temperatures and the accumulation of corrosive particulates. Because these regions had previously been exposed to temperatures and conditions at which environmental damage did not have a tendency to occur, protective coatings were not generally required. However, as today's turbine blades continue to be exposed to increasingly higher operating temperatures, particulates accumulated on the surface have started to melt and cause type II hot corrosion attack, which can lead to premature failure of the turbine blade. Type II hot corrosion conditions generally require a chromium diffusion coating instead of a diffusion aluminide coating for protection.
The vanes are subject to similar attack to the blades, as the vanes are generally made from similar materials to the blades, and also may have cooling channels.
As can be seen, different regions of a turbine blade are susceptible to different types of damages. Adequate protection therefore requires selectively applying different protective coating systems to various components of the turbine blade. In particular, applying chromizing coatings locally onto only those regions of the turbine blade susceptible to hot corrosion attack is required.
However, conventional coating processes have their limitations for successfully applying chromizing coatings onto only selected regions of the component. For instance, conventional chromizing processes, such as pack chromizing and vapor phase chromizing, are not capable of forming a chromium diffusion coating onto selective regions of a turbine component without utilizing a customized diffusion-stop-off masking apparatus or post-coating treatment. Diffusion-stop-off masking is defined as the apparatus or technique which is used to prevent the chromium diffusion into substrate surface where no chromium diffusion coating is required.
Pack chromizing processes require a powder mixture including (a) a metallic source of chromium, (b) a vaporizable halide activator, and (c) an inert filler material such as aluminum oxide. Parts to be coated are first entirely encased in the pack materials and then enclosed in a sealed chamber or retort. The retort is then heated in a protective atmosphere to a temperature between about 1400-2100° F. for about 2-10 hours to allow Cr to diffuse into the surface. However, a complex and customized diffusion-stop-off masking apparatus is required to prevent chromide coating deposition at desired locations. Furthermore, pack chromizing processes require an in-contact relation between the chromium source and the metallic substrate. Pack chromizing is generally not effective to coat inaccessible or hard-to-reach regions, such as the surfaces of internal cooling passages of turbine blades. Moreover, undesirable residuals coatings can form. These residual coatings are difficult to remove from the cooling air holes and internal passages, and restriction of air flow may occur. Therefore, pack chromizing is not effective to selectively coat the surfaces of the internal cooling passages.
Vapor phase chromizing processes are also problematic. A vapor phase chromizing process involves placing the parts to be coated in a retort in an out-of-contact relationship with a chromium source and halide activator. Although a vapor phase process can effectively coat the surface of internal cooling passages, the entire surface is undesirably coated. As a result, the turbine blade needs to be masked along those regions where no chromizing coating is required. However, masking is challenging and often does not entirely conceal regions of the blade intended to be masked. Consequently, special post-coating treatments such as machining, grit blasting, or chemical treatments are required to remove the excess chromizing coating where no chromizing coating is required. Such post-coating treatments are generally non-selective and result in undesirable loss of the substrate material. The material loss can lead to changes in critical dimensions of turbine components and lead to premature structural dimension. Additionally, special care is typically required during post-coating treatments to prevent damage to the substrate or any chromizing coating not removed.
The problems of utilizing a pack or vapor phase chromizing process are exacerbated as the geometry of certain components of the turbine component become more complex, such as the regions under the platform, shank, root and internal cooling passages.
In view of the drawbacks of existing chromizing processes, there is a need for a new generation chromizing process that can produce a chromizing coating in a controlled and accurate manner on selective regions of a component, thereby minimizing masking requirements for areas where no coatings are required, reducing material waste and raw material consumption and minimizing exposure to hazardous materials in the workplace. Other advantages and applications of the present invention will become apparent to one of ordinary skill in the art.